In the world of engineering, it is all about making machines that can work even in the most difficult situations.
Autorotation is a very important maneuver for helicopters that can mean the difference between life and death.
I will explain what autorotation is, how it works, and why it is so important for helicopter pilots and engineers in this blog post.
So, get ready to learn about one of the most important parts of flying a helicopter.
Introduction to Autorotation in Engineering
Formal definition:
1. Rotation about any axis of a body that is symmetrical and exposed to a uniform airstream and maintained only by aerodynamic moments. 2. Rotation of a stalled symmetrical airfoil parallel to the direction of the wind.
Autorotation is a type of flight in which the main rotor system of a helicopter or other rotary-wing aircraft turns without being powered by the engine.
This is similar to how an autogyro works.
When the engine or tail-rotor stops working, this method is often used to land the helicopter quickly. But it can also be used to get out of a vortex ring and as a training tool when a pilot is learning how to fly.
How Autorotation Works
During autorotation, the pilot disconnects the engine from the main rotor system. This lets the upward flow of air drive the rotor blades alone.
To control the autorotation RPM, the pilot changes the size of the autorotation area in relation to the driven and stall areas.
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Aerodynamics of Autorotation
Autorotation is a very important emergency procedure in helicopter flight. It lets the main rotor of a helicopter move only because of air pressure, not because of the engine.
Variables Affecting Autorotation
The main things that affect how well autorotation works are:
- Density Altitude: At high density altitudes, where the air is less dense, the rate of descent will be faster.
- Gross Weight: Helicopters with more weight fall faster.
- Airspeed: The pilot has the most control over the rate of descent during autorotation through airspeed.
Just like in normal flight, the cyclic pitch control makes the plane go faster or slower.
Autorotative descents at very low or very high airspeeds are more dangerous than those done at the minimum rate of descent airspeed.
- Rotor Rotational Speed: As rotor rotational speed goes up, the rate of descent goes down.
Driving Region of Autorotation
During autorotation, the driving region or autorotative region is usually between 25 and 70% of the radius of the blade. This is where the forces are made that turn the blades.
There is an angle to the total aerodynamic force in this area.
Flare Landing and Energy Absorption
When landing from an autorotation, the kinetic energy stored in the rotating blades and the forward movement of the plane are used to slow the rate of descent and make a soft landing.
A higher rate of descent means that more rotor energy is needed to stop a helicopter than a lower rate.
Maneuvers in Autorotation
When a plane loses power, a pilot must do three important things:
- Autorotation: This maneuver covers the change from normal powered flight to steady autorotation.
- Steady Autorotation: During steady autorotation, the engine's aerodynamic forces alone must result in zero net torque.
Because the plane is going down, air flows up through the main rotor. The shape of the blades also makes this easier.
The main rotor is connected to the tail rotor with gears, so in normal flight, the main rotor drives the tail rotor.
But during steady-state autorotative descent, when the engine loses power and torque drops to zero, the tail rotor stops working as an anti-torque device because it gets torque from the main rotor system through the transmission system.
- Flare Landing: In this maneuver, the spinning blades and the forward movement of the plane are used to slow the rate of descent and make a soft landing.
- Touch-Down - The vehicle is gently landed using the energy remaining in the rotor head.
Flight Manual Airspeed Limitation for Autorotation
In autorotation, there will be a speed above which the parts of the rotor blades that drag behind the rotor will extend so far along the blade span that the rotor will start to slow down a lot.
This airspeed is usually written as the maximum airspeed allowed for autorotation in the flight manual.
Helicopter Autorotation Maneuver
Basic Autorotation and its Four Sections
There are four parts to the basic autorotation:
- Glide: During this part, the helicopter is in a stable autorotative descent, and the pilot changes the flight path by turning the helicopter or changing the airspeed.
- Flare: In this section, the rate of descent is slowed by using the kinetic energy stored in the rotating blades and the forward movement of the plane.
- Landing or Power Recovery: In the last part, the helicopter either lands softly or the pilot gives it power to get back up.
Practical Application and Advanced Autorotations
The real-world use of autorotation training is similar to what pilots do when they practice forced landings without power.
Just like in an airplane, all the helicopter pilot has to do to start a go-around is put power back on. But it is very important to be accurate and know how to move the helicopter when it is in autorotation.
Collective for Rotor RPM Control
Pilots of helicopters need to know how to use the collective to control the speed of the rotors during power-off autorotations in a turn.
When the collective is moved up, the rotor's RPM goes up, and when it is moved down, the RPM goes down.
Safety Limits and Risks
There are risks that come with doing autorotations during training.
In the last part of an autorotation, the helicopter's kinetic energy may run out, leaving it with little or no cushioning effect
This could lead to a hard landing that damages the helicopter.
The aircraft height versus speed diagram tells us what the safest way to do this maneuver is.
Modeling and Simulation of Autorotation
Simulations and models on computers have become popular ways to study and improve the performance of autorotation in helicopters.
Computer simulations can be used to figure out how different helicopter designs or rotor blade shapes affect how well the helicopter can fly on its own.
Researchers have also made and tested in a simulator a number of pilot cues that will make it easier for the pilot to control the helicopter when it is rotating on its own.
During autorotation, a set of discrete and continuous cues are defined to help the pilot know what is going on and what to do.
Benefits of computer simulation modeling
Computer simulation modeling is useful when making changes to the real system is hard, expensive, or just not a good idea.
It uses computer software to model a real or proposed system, and designers, program managers, analysts, and engineers use it to understand and evaluate "what-if" case scenarios.
For example, instead of actually crashing dozens of new cars, car companies use computer simulations to test their new lines of vehicles.
Limitations of computer simulation modeling
In general, one of the problems with computer models is that they can not accurately take into account all the factors that could affect how a system works.
This is especially true when trying to understand complicated aerodynamic phenomena, like how a helicopter can turn on its own.
Another thing that needs to be looked at is how computer simulations affect the general public. So, using modeling and simulation without being careful could lead to wrong conclusions.
Some rules, like figuring out where a defense system is not working right, need to be taken into account when judging the validity of any simulated system.
In the end, computer simulations have many benefits, but they also have some problems that need to be carefully thought through before any conclusions are made.
Real-World Examples of Autorotation
Real-world examples of autorotation show how important it is for helicopter pilots to know how to do this maneuver in case of an emergency.
Robinson Heli Down Autorotation
The Robinson Heli Down Autorotation is a way for a helicopter to land safely if its engine stops working.
Robinson helicopter pilots often use it as part of their training, and here are the steps:
- The pilot must first realize that the engine has stopped working and then start the autorotation maneuver right away by lowering the collective and going into an autorotation descent.
- To get to a safe landing zone, the pilot must then set a steady rate of descent and keep the speed of the rotors steady while turning.
- During the descent, the pilot should keep an eye on the airspeed and rotor rpm and use the cyclic pitch control to make changes as needed while keeping control of the plane.
- When the helicopter gets close to the ground, the pilot should turn it sideways to soften the landing and slow its rate of descent.
Autorotation in Drones
Autorotation is useful not only for full-size helicopters, but also for remote-controlled helicopters, and drones.
The idea behind autorotation has not changed: the main rotor turns because air moves up through it, rather than because the engine turns it.
Autorotation in Remote-Controlled Helicopters and Drones
Most collective pitch remote-controlled (RC) helicopters can also auto if the engine flames out or the motor stops working for some reason.
In order for the helicopter to do autorotations, the main rotor shaft needs to be able to be taken out of the rest of the drive or gear assembly.
Some small drones may use a small electric motor to keep the rotor spinning at the same speed during autorotation, while others may rely only on the forces of air to keep the rotor spinning at the same speed.
For remote-controlled helicopters and drones, the key to a successful autorotation landing is to keep the rotor speed constant and use the pitch, cyclic, and collective controls to control the aircraft's descent and forward speed.
For remote-controlled helicopters and drones, the best way to do autorotation landings is to practice in a safe, open area away from people, buildings, and other obstacles, and to gradually make the maneuvers more difficult as your skills improve.
Also, it is important to keep the plane in good shape and do regular maintenance and inspections to make sure it works well. Pilots should follow all laws and rules about how to operate remote-controlled aircraft.
The VECTOR autopilot, which was made by UAV Navigation, is one of only a small number of autopilots that can fully rotate on its own.
To get good at autorotations, it is important to practice them often in a safe place.
The key to a successful autorotation landing is timing and knowing when to slow your descent and forward speed by flaring with a rear cyclic command, applying positive collective pitch, and then leveling out the aircraft just before touch down with forward cyclic to land gently.
It is also important to choose a good place to land and get close to it at the right angle and speed.
Video and references
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https://en.wikipedia.org/wiki/Autorotation
Use cases
| Used in: | Description: |
|---|---|
| Emergency landing | One of the most important ways autorotation is used is when a helicopter's engine stops working in an emergency. When this happens, the pilot can start autorotation, which makes it possible for the helicopter to safely glide to the ground. This move can save the lives of both the pilot and the people on board. |
| Military | Autorotation is a useful skill for military helicopter pilots who are in charge of military operations. In combat, a helicopter may need to quickly drop to the ground to avoid getting hit by enemy fire. By using autorotation, the pilot can quickly and safely land the helicopter. |
| Search and rescue | Helicopters are often used for search and rescue, especially in hard-to-reach or remote areas. In these kinds of situations, autorotation can help the helicopter land safely and in control, even on rough terrain. |
| Agriculture | Autorotation can also be used in farming, especially when dusting crops. |
| Filming and photography | Helicopters are often used to get shots from above in the film and photography industries. |
| Maintenance of power lines | Sometimes, helicopters are used to do maintenance on power lines, like fixing or replacing broken lines. Autorotation can help the helicopter land safely and accurately in places that are hard to get to any other way. |
Conclusion
In conclusion, the idea of autorotation shows how powerful engineering can be and how creative people can be.
It gives us the confidence to fly in helicopters and other planes, knowing that if the engine stops working, we can still safely glide back to the ground.
Autorotation also shows us that when we push the limits of what is possible, we can do amazing things.
Engineers and pilots have always been driven to find new and better ways to fly, from the first times people flew to the cutting-edge technology used in modern aviation.
Autorotation is just one of the amazing things we can do when we put our minds to it.
So, the next time you look up and see a helicopter flying, think about autorotation, a feat of engineering that makes it all possible.
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